1. Field of the Invention
The present invention relates to an electrophotographic printer, and more particularly, it relates to an electrophotographic printer which forms an electrostatic latent image on a photosensitive belt.
2. Description of the Prior Art
In this type of the electrophotographic printer, the position of a photosensitive belt shifts in the width direction of the belt according to the rotation of the photosensitive belt, and therefore, previously, steering control has been performed to compensate this shift. The steering control means such a control that a positional shift of the photosensitive belt is detected and inclination of a roller to rotate the belt is changed according to a shifting amount and the belt is set at a fixed position.
A conventional example is shown in FIG. 9. In FIG. 9(a), a photosensitive belt 51 is hung over a roller 52 with a proper tension. Here, there are a plurality of rollers over which the belt 51 is hung, but part of them are omitted in the figure. Near the photosensitive belt 51, a laser output section 53 is provided. This laser output section 53 has a function to radiate a laser beam on the surface of the belt 51 and to make this laser beam scan in the width direction of the belt 51. This laser beam scanned by the laser output section 53 is not only radiated on the surface of the belt 51, but also radiated on positions apart from the belt 51 at the edge of the scanning range thereof. This laser beam slipping off the belt 51 forms a light spot on a mask 54 according to the scanning position. This mask 54 is provided opposite to the laser output section 53 through the belt 51. Furthermore, the positioning of each component is made so that the state may gradually change among a state where this laser beam does not come in both the belt 51 and the mask 54, a state where the laser beam comes in only the mask 54, a state where the laser beam comes in both the belt 51 and the mask 54, and a state where the laser beam comes in only the belt 51 in the course of scanning of the laser beam when the belt 51 is arranged in a normal position.
This state will be described referring to FIG. 9(b). FIG. 9(b) is a figure in which the belt 51 and the mask 54 are seen from the laser output section 53 side. Now, when the light spot is scanned in the direction of numeral 56a.fwdarw.56b.fwdarw.56c, the light spot does not come in both the belt 51 and the mask 54 in the state of numeral 56a. Next, in the state of numeral 56b, the state becomes a state where the light spot comes in both the belt 51 and the mask 54 through a state where the laser beam comes in only the mask 54. Moreover, in the state of numeral 56c, the state becomes a state where the light spot comes in only the belt 51 but it does not come in the mask 54. Accordingly, when a laser beam is scanned by 1 line, the laser beam comes in the mask 54 only for a short time, and since the scanning speed of the laser beam is constant, time of coming-in changes according to the position of the belt 51. For example, in FIG. 9(b), when the edge of the belt 51 moves from a solid line to a two-dot-chain line as shown by arrows, the time when the laser beam comes in the mask 54 becomes shorter. Therefore, by detecting this light spot coming in the mask 54 by using a photo sensor 55 in FIG. 9(a), the position of the belt 51 can be detected from the time when output of this photo sensor 55 is turned on.
Previously, feedback control of the position in the width direction of the belt 51 has been performed in such a way that a positional shift of the belt 51 is detected by using a position detecting means like this and the roller 52 is inclined according to the shifting amount.